Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Fields -- was All that glisters

On Fri, 12 Feb 1999, Jim Green wrote:

Folks this is thread has devolved into one of the various mythologies of
mis-used language: Let us remember that "fields" don't really exist; they
are inventions -- mathematical constructs if you will -- which are supposed
to make our calculations easier. Like many such constructs in other
places, it is OK to use anthropological language but only as long as one
realizes what it rally happening.

Hi Jim! I would agree that "lines of force" are a simply a helpful mental
aid, and they have no real existence.

But Electromagnetic energy itself is "real", in the same way that a light
beam or a radio wave is "real." If "Something" causes the iron filings
around a magnet to align, we should not doubt the existence of that
"Something," even though we cannot perceive it directly with our eyes.

A reference: FIELD ENERGY AND FIELD MOMENTUM, The Feynman Lectures on
Physics, ch 27, vol. II

Same with grass-seeds which align when placed between the plates of a
capacitor with high P.D. between its plates: Something is there in empty
space, although it does not refect light and hence is not visible. We
might mentally-model this "Something" as a population of (virtual?)
photons, or as e-fields and/or b-fields. But just because the "Something"
is invisible, or because there are more than one model used to describe
it, this does not force us to declare that it is nothing but an abstract
concept.


|... If we look at the field lines around an accelerating electron in
|a field, we find changing e-field, which is the same as circular b-field,
|which, when crossed with the static e-field, gives an inward-directed
|Poynting vector field.

Well maybe yes, but remember that to calculate what happens to the
electron, one considers _only_ a "field" due to all _other_ sources -- It
makes no sense to talk about an electron which is moving in a field due to
it's own charge.

Right! When I say "an electron accelerating in a field", I mean that this
"field" is a parallel field in the environment, a field created by some
distant charges somewhere else. (They could be VERY distant, as with the
fields in the solar system. In that case, the electron cannot communicate
instantaneously with the distant charges, it cannot instantly apply
mechanical reaction forces upon them, therefor, when a parallel field in
empty space causes an electron to accelerate, the increase of the KE of
the electron must be "fueled" only by the energy stored in local fields in
the surrounding space.)

|The electron is "sucking energy" from the e-field
|as it accelerates, and the e-field is slightly weaker after the electron
|has passed. The poynting-vector field seems to paint a clear picture of
|how this energy is moving around.

I have no idea what "sucking energy" might mean. The _only_ way for an
electron to gain energy is for something to do work on the electron.

Correct. The parallel e-field in space can directly perform work upon the
electron. Fields also carry momentum, as Feynman showed with his infamous
rotating ring of charged spheres (Feynman Lectures Vol. II, Fig. 17-5 on
p. 17-6)

Energy is not just floating about to be "sucked".

I would disagree. An e-field contains energy U= 1/2 * k * (V/D)^2 in each
unit volume of space. (V/D is field strength in volts per meter)
Therefor, energy IS floating around waiting to be "sucked." In my
opinion, an e-field itself CAN do work upon an electron, and this concept
becomes a bit clearer if we imagine that the source-charges of the
environmental e-field are a light-year away, so that only the e-field in
the space local to the electron can do work upon the electron.


The energy present in e-fields? Let's see. I recently figured out how to
calculate the fields and energy in a capacitor, so like a kid with a new
hammer, I've just got to use it to pound upon everything which comes
along: :)

Capacitance = 8.9 * 10^-12 * platearea / plategap (MKS units)

Total Energy in the capacitor = .5 * Capacitance * Volts^2

Volts between plates = fieldstrength * plategap


So, for a capacitor 1 meter on a side (imagine parallel fields with no
fringing effects), the Capacitance is 8.9 picofarads, and the energy per
cubic meter in the empty space between the capacitor plates depends upon
the square of the intensity of the e-field, and we can calculate the
energy stored in a volume of space which contains an e-field:

Energy/meter^3 (joules) = 8.9/2 * 10^-12 * fieldstrength^2

An e-field of 1 volt/meter will store 4.5*10^-12 joules in each cubic
meter occupied by that field. (Note that the e-fields which surround
everyday electric circuits are MUCH stronger than 1 volt/meter, and so the
density of the EM energy is MUCH higher.)

With an electron in space, where this space contains a parallel e-field,
the strength of the e-field near the electron does not form a spherical
pattern, since the e-field of the electron is directed opposite to the
e-field of the environment in some spots, causing the two fields to
partially cancel there. In other spots, the fields reinforce. If we
watch an electron in a parallel e-field as it accelerates past us, the
environmental e-field was stronger before the electron approached, and
weaker after the electron has passed by. Let me try to sketch this in
ASCII:

___________________
__--
___-- ___-----------------
_---- _-- _________________
___-- /
___-- | _--------------- The dot is the electron
\./________________
____ / \_
---__ | ---------------
_ -_ \ _________________
----___ --___
--__ -----------------
--___________________

The above electron would accelerate from left to right, with more energy
on the right side of the diagram and less on the left. (I've found this
mental picture to have very wide application! Think of the charges inside
of resistors. Think of the creation of more flux-lines between the plates
of a capacitor whenever we force electrons to flow across the gap, forcing
them to move against the existing e-field.)

If we look at the poynting-vector field of the accelerating electron, we
see energy being extracted from space and flowing into the electron,
increasing its KE. Therefor the electron "sucked energy" out of the
parallel e-field, and hence was accelerated.

(Yes, I'm probably distorting some Relativity concepts to the breaking
point here. What happens if the observer is moving relative to that
pre-existing parallel e-field? Does the electron still "suck energy" out
of its environment? Does the energy still "have" a specific location? Do
paradoxes appear? Feynman mentions this issue on p. 27-6 of Vol.II of the
Feynman Lectures.)

|When the electron is driven by the field, energy flows out of the e-field
|and into the electron (the flow looks like it has a contracting torus
|shape surrounding the electron, but perhaps it is actually spherical?)

It makes no sense to me to say that "energy flows out of the E-field.
Perhaps others can explain this to me and use terms that make sense.
Certainly there is no "energy" _in_ the field to flow anywhere.

No energy in an e-field? Uh oh.

If our viewpoints are THIS different, then I see that we cannot
communicate at all!

In my view, an e-field is MADE of electromagnetic energy and little else.
It is a cloud of photons. It is the electrical component of the "stuff"
known by the name "electromagnetic energy."


|When the electron drives against the field and deccelerates (such as after
|a classical electron has bounced backwards from a lattice vibration or an
|ion), then the electron is dumping energy into the field as it slows down.
|(These backwards-moving electrons would charge up the entire circuit,
|while the forwards-moving ones cause the whole circuit to discharge.)

"Dumping energy" -- one could say "doing work", but one would have to say
how that work is done.

If we only have fields in space, with no other objects upon which to
perform mechanical work, then where does the KE go when an electron
decellerates? (Again, imagine that the source-charges of the parallel
field in the environment is many light-years distant, and so we can ignore
it, and concentrate exclusively upon the local e-fields.) I would say
that the energy goes into "lengthening of flux lines", or more accurately,
a larger region of e-field is created as the electron decelerates,
therefor the KE of the electron is being lost, and the EM enery in the
field is rising.

|> That's why I like to invoke an electromagnetic field
|> and the poynting vector (as apparently William Beatty does,
|> judging by his contribution where he describes this model in
|> excellent detail).
|
|I didn't start it, I got it from a fairly old textbook which had a

We all should have learned by now that "old textbooks" are full of
mythology and mythological language and hand waving. Sometimes this hand
waving solves engineering problems -- often it does not.

Are my "energy sucking" mental models mythological or real?

Don't trust *MY* word, and don't trust the old textbooks. Go read the
Feynman Lectures, which talk about just these issues. Let's see... I flip
around and I find page 27 Fig. 27-3, which shows the inward-pointing
Poynting-vector field surrounding a capacitor being "charged." Lots more
stuff like that there.


Etc, etc, etc etc.

Perhaps I am wrong in all of the above, and you should rightly erect
mental barriers to prevent this concept-infection from spreading from my
mind to yours.

However, be careful about dismissing this stuff out of hand. Does it
sound crazy? Please, go read Feynman's chapter 27 about EM field enery
and field momentum, and then decide.

As far as I can tell, these ideas are not crazy, instead they are totally
in line with very orthodox physics. (However, I've yet to see them in
many places other than THE FEYNMAN LECTURES, and that old physics book I
stumbled across in 1985 or so.)

If all of the following is just a system of misconceptions, then I would
request that people here try to discover the flaw that makes the whole
thing collapse. I've been using these concepts for awhile, and don't see
obvious holes. However, I'm NOT a physics teacher. If I tried to teach
these concepts to students, perhaps I'd hit the big pothole which
demonstrates the fragility of this "vehicle." At the very least, please
show me how my mental model differs from the one presented in the Feynman
Lectures' treatment of E & M.

Jim, since I see no more comments in the following, I'm assuming that
you're dismissing the whole thing. In my opinion, this is a very serious
mistake. These "visual" electricity/energy concepts are valuable in the
extreme. PLEASE PLEASE PLEASE go read Feynman's chapters on this topic!
It is not just me and the dusty textbooks who describes EM fields in this
way.

|treatment of the poynting-vector field surrounding a resistor. The EM
|energy in that situation contracts inwards around the resistor, like a
|shrinking cylinder-shape. If the wires are like 1-ohm resistors
|themselves, then we can draw a picture of the "lines" of energy flow. If
|we draw a schematic diagram, then we sketch in the e-field lines which
|extend between different parts of the circuit, then it becomes trivial to
|sketch in the "field lines" of the energy flow. The poynting-vector
|"lines" are simply the field that's perpendicular to the e-field in our
|2-dimensional sketch. Imagine a "charged" capacitor that's heating a
|resistor. The poynting-vector "stuff" comes flowing out of the dielectric
|of the capacitor, pours along through the empty space that's surrounding
|the pair of conductors, then dives into the resistor, entering its
|surface perpendicularly from all directions.
|
|I was forced to come up with this mental image because I wanted to answer
|the question "what doees electricity look like?" If "Electricity" equals
|EM energy, then we can sketch a diagram of the flow of "Electricity" in
|the space surrounding a DC circuit (and then quickly notice that this is
|just like conventional RF twinlead waveguide stuff, but operating at DC!)
|
|Weird delerious vision: our entire civilization is run by radio waves.
|Low frequency radio waves. Guided by twinlead waveguides. At 60Hz. :)
|(what would happen if an AC electric generator was connected to an
|enormous dipole antenna, with 1/4 wavelength dimensions at 60Hz? The
|"electricity" energy-stuff would flow from the generator, run along the
|connecting wires, then spew right out into space!)
|
|Some of these issues are clearer if AC is involved: the electrons
|generally sit in one place and wiggle, somewhat similar to air molecules
|during a sound wave. Do the electrons also move fast? Yes, but then so
|do the air molecules. Judging from microscopic examination of air
|molecules, we might predict that human being experience constant fierce
|hurricain-like winds. Instead we just experience air pressure and gas
|diffusion phenomeona. I like to imagine that electric circuitry is
|similar: the Drift Velocity is akin to wind or to water flow. The actual
|particle velocity is akin to the Brownian Motion movements of the fluid,
|and is usually ignored when we work at a macroscopic scale. High velocity
|electrons do store "heat", and they might create electrical white-noise,
|but at the same time they behave as if they are unmoving, just as a parcel
|of air behaves as if it is unmoving when the wind does not blow.
|
|((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
|William J. Beaty SCIENCE HOBBYIST website
|billb@eskimo.com http://www.amasci.com
|EE/programmer/sci-exhibits science projects, tesla, weird science
|Seattle, WA 206-781-3320 freenrg-L taoshum-L vortex-L webhead-L


Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen


((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
Seattle, WA 206-781-3320 freenrg-L taoshum-L vortex-L webhead-L